PURPOSE: To shorten a voltage ratio measuring time by reducing a multiplication processing amount, by comparing the predetermined numbers of sampled vector and voltage while subjecting the same to A/D conversion.

CONSTITUTION: When AC voltage Ea satisfying formula I (wherein M and N are integers) of a cycle Ti forming a vector is sampled by a sampling cycle Ts satisfying formula II, the sampled data value P comes to a value satisfying formula III and the component Eax in the x-direction of the vector satisfies formula IV. Therefore, when the numbers N of samples are set to integral multiple of 4, the absolute value of the formula IV becomes equal when h=1, 3, 5, 7 and h=2, 6 and the processing numbers of multiplications are reduced to 1/4. It is similar even with respect to a y-direction component and vector to voltage ratio measuring time is shortened.